This invention in general relates to optical communications and in particular to resonant structures useful as components for a variety of applications in optical communication systems and, as well, for sensing applications.
Structures resonant in the optical region of the spectrum are well-known in the art having been disclosed in both fiber and integrated optics formats. Known applications for these structures include their use as rotation sensors in gyroscopes, spectrum analyzers, and optical filters.
Whether in fiber or integrated format, a resonant structure includes a waveguide in the form of a continuous optical path, which may be closed ring, and some means for coupling energy in and out of that path. When used for filtering applications, the two most important characteristics for such structures are their free spectral range and finesse.
Free spectral range refers to the frequency difference between adjacent resonant orders of the structure. In fiber optic communication systems, where such structures are to be used as filters that allow optical signals to be placed on the bus without deliteriously affecting the transmission of other wavelength signals travelling by the filter on the bus or as filters that extract a selected wavelength from the bus while allowing other wavelengths to pass by the filter with low attenuation, the capacity of such filters, referred to as bus tap ins or offs, is equal to the free spectral range of the resonant structure. That is, signals travelling on the bus at frequencies equal to each other within the free spectral range cannot be separated from one another using this type of filter. Both will either pass by the resonator or enter the resonator, depending on whether the resonator is not tuned or is tuned to one of the pair. Therefore, a large free spectral range is desirable as a means for providing a high channel capacity through a wavelength multiplexed communication bus.
Finesse measures the ratio of the free spectral range to the half-height optical signal bandpass of the resonant structure. Bandpass, in turn, indicates the capacity of the single channel of a communication link. For example, a suppressed carrier, single sideband signal having a capacity of transmitting N data bits per second requires a bandpass of approximately N Hz. Thus, the useful channel capacity is equal to the bandpass of the resonator and is calculated from the free spectral range and finesse of the resonator. If, for example, the resonator has a free spectral range of 10 Gz and a finesse of 100, then the resonator channel bandwidth is 100 MHz and a signal of at most this bandwidth may pass through the resonator.
Finesse also provides a measure of the signal to cross-talk ratio of an optical wavelength multiplexed system using resonant structures as bus tap filters. For relatively high finesse resonators (greater than 10), the strength of a signal passed through a ring resonator filter at resonance is larger, by a factor approximately equal to the square of the finesse, than the signal passed through when tuned midway between resonances. Moreover, if a communication link carries simultaneously a number of signals, separated by equal amounts in frequency equal to the value of the finesse, then the fully populated link signal to cross-talk ratio is substantially equal to the finesse.
Based on these considerations, it is clearly desirable to construct resonant structures having as high as possible a value of finesse that is consistent with the desired channel capacity for an optical communication link channel.
Resonant structures in fiber form have been fabricated by forming a loop and having regions of the fiber loop adjacent its free ends optically coupled with a suitable bulk coupling means such as that shown and described in U.S. Pat. No. 4,469,397 issued to Herbert J. Shaw et al on Sept. 4, 1984 and entitled "Fiber Optic Resonator".
Integrated optic resonant structures have been fabricated utilizing photolithographic techniques known in the semiconductor industry along with ion diffusion processes involving a variety of material systems. R. G. Walker and C. D. W. Wilkinson, for example, disclosed on Apr. 1, 1983 in an article entitled "Integrated Optical Ring Resonators Made by Silver Ion-exchange in Glass" in Vol. 22, No. 7, Applied Optics, how to fabricate an integrated optic ring resonator by silver ion diffusion in soft glass, silver and sodium substituting for one another. Others have shown that lightguide structures can be fabricated on the surface of soft glass via potassium or lithium substitution of sodium.
Kazuo Honda et al have also described the structure of an integrated optic ring resonator. Their resonators, which were disclosed in the Journal of Lightwave Technology in Vol. LT-2, No. 5 in October, 1984, were multimode in nature and were characterized by relatively high loss and relatively low finesse. Because the structures disclosed support propogation of multiple modes, they are not considered to be desirable for communications purposes since the free spectral range of the resonator is in effect reduced by the presence of additional resonances within one free spectral range.
Recently, K. H. Tietgen described, at the seventh topical meeting on integrated and guided wave optics held on April 1984 in Kissimmee, Fla., the fabrication of a ring-like resonator by titanium diffusion in lithium niobate. A desirable feature of this ring is the fact that the substrate material is electro-optic.
In spite of the many innovations made in this art, improved resonant structures are still required and can be usefully employed in optical fiber communication systems for a variety of purposes. In particular, resonant structures of high finesse, large free spectral range, low susceptibility to optical damage, and tunability can be used to advantage in modulation and filtering applications and as well, in a variety of environments requiring sensors. Furthermore, ease and efficiency of manufacture and the use of electro-optic materials are important considerations. Accordingly, it is a primary object of the present invention to provide resonant structures having the above beneficial characteristics and which can be used for a variety of purposes.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the structure and method exemplified in the detailed disclosure which follows.